Novel electronic gas and photo sensors assisted via zeolitic imidazolate framework

Abstract

Zeolitic imidazolate frameworks (ZIFs), containing abundant micropores for selective passage and adsorption of specific molecules, have attracted tremendous attention for gas storages, gas separators, catalytic systems, or energy storage systems. Very recently, researchers are trying to exploit the materials into electronic sensors for realizing unprecedented functions such as extraordinary selectivity for gas detections. However, inserting the ZIFs into electronic systems are still in the early stages owing to its low electrical conductivity that causes undesirable electrical noises when it is adopted as a channel for conventional two-terminal or three-terminal devices. Therefore, a study on exploring an optimal sensor design is a prerequisite for the effective utilization of ZIFs for electronic sensors. This thesis suggests novel electronic sensors (gas sensors and photodetectors) comprising ZIFs and strategies for achieving high-performance and new functionalities simultaneously. This thesis begins with an overview of electronic hydrogen (H2) gas sensors, and UV photodetectors in chapter 1, in which general device structures, working mechanisms and the current state-of-art technologies with their limitations are described. Besides, a review of the recent advancement of ZIF-based sensors is accompanied in the same chapter. The first topic of this thesis (Chapter 2) is about battery-free hydrogen (H2) sensors, operated in photovoltaic mode; a diode based on n-type metal oxide and p-type light absorber with palladium (Pd) catalyst is utilized as a photovoltaic gas sensor. Herein, ZIF-8, a class of ZIFs, is utilized as a molecular sieve covering the sensor not only for accumulating H2 gas at the vicinity of the sensing segment but also for preventing surface contamination or physical damage of the Pd catalyst – resulting in enhancement of sensitivity and long-term stability. In the following chapter (Chapter 3), Poly(Methyl Methacrylate) (PMMA) is exploited for improving gas selectivity of ZIF-based molecular sieve. The PMMA-modified ZIF layer is capable of filtering out carbon monoxide that causes fatal damage on Pd catalysts in H2 sensors, as well as accumulating H2 at the vicinity of the catalyst selectively. Consequently, the photovoltaic H2 sensor covered with the new molecular sieve demonstrates superior sensing performances (response time < 10 s and sensing response > 5,000% at 1% H2) and outstanding stability even at the exposure of 1000 ppm CO gas. The last chapter (Chapter 4) of this thesis deals with novel utilization of ZIFs for zinc oxide (ZnO)-based ultraviolet (UV) detector. One of the findings is a suppression of surface defects on ZnO nanomaterials when their surfaces are converted into ZIF-8. The changes make ZnO-based detectors to be more beneficial for depressing a defect-induced visible light detection, as well as to respond to UV light with accelerated sensing speed. Moreover, although the ZIF-8 with large bandgap (> 5 eV) is not photoactive materials for UV light, their high lowest unoccupied molecular (LUMO) level implies potential use as an electron blocking layer, which effectively reduces recombination probability of electron-hole pairs. Consequently, the surface conversion of ZnO into ZIF-8 realizes a UV detector showing 6.5 times enhanced responsivity and 20 times faster sensing speed than those of the bare ZnO detectors.